Electric motor and reduction motor

ABSTRACT

Disclosed is a motor in which a commutator ( 10 ) is provided with connecting wires which short-circuit equipotential segments; brushes ( 21 ) are constituted by a low-speed brush ( 21   a ), a high-speed brush ( 21   b ), and a common brush ( 21   c ) used in common by the low-speed and high-speed brushes, and are juxtaposed along the circumferential direction. The circumferential brush width (W 2 ) of the high-speed brush is set to be smaller than the circumferential brush width (W 1 ) of the low-speed brush. The high-speed brush and the low-speed brush are formed so that simultaneous sliding contact with equipotential segments ( 15 ) can be avoided. Additionally, armature cores ( 8 ) are provided such that a plurality of teeth ( 12 ) is point-symmetrical about a rotary shaft ( 3 ) at equal intervals in the circumferential direction, and the teeth and slots ( 13 ) are formed so as to exist alternately at intervals of 90 degrees in the circumferential direction. By virtue of the above configuration, vibration and noise can be reduced while achieving miniaturization and high performance of a motor.

TECHNICAL FIELD

The present invention relates to an electric motor mounted on, forexample, a vehicle, and a reduction motor using the same.

Priority is claimed on Japanese Patent Application No. 2008-260987 filedon Oct. 7, 2008, the contents of which are incorporated herein byreference.

BACKGROUND ART

Electric motors with brushes have conventionally often been used aswiper motors for an automobile. In this kind of electric motor, aplurality of permanent magnets is arranged at equal intervals in thecircumferential direction at the inner peripheral surface of acylindrical yoke, and an armature is rotationally supported inside thesepermanent magnets. The armature has an armature core in which aplurality of teeth is formed in a radial fashion. A plurality of longslots is formed in the axial direction between the respective teeth, andcoils are formed by coiling winding wires between the slots withpredetermined intervals by an overlapping winding method. The coils areelectrically connected to a commutator which is fitted and fitted to therotary shaft from the outside so as to be adjacent to the armature core.

The commutator has a plurality of segments, which is metal pieces,arranged in the circumferential direction in a mutually insulated state,and a winding starting end and a winding finishing end of a coil areconnected to each segment. Additionally, the segments are connected tobrushes, respectively, so as to be capable of sliding contact with thesegments, and electric power is supplied to the respective coils via thebrushes. Also, a magnetic field is formed in a coil to which electricpower is supplied, and a rotary shaft is driven by a magnetic attractiveor repulsive force which is generated between the permanent magnets ofthe yoke.

Here, recently, from necessities of small size and high performance ofthe wiper motor, there is disclosed a technique for achieving multiplepoles in which the number of magnetic poles is 4 (the number of polepairs is 2) and multiple slots, achieving high performance in a motor,arranging four brushes at equal intervals in the circumferentialdirection, and making the speed of the motor variable (for example,refer to Patent Document 1).

In the motor of Patent Document 1, the amount of an electric current tobe supplied to a coil in each mode of a LOW mode, a MID mode, and a HImode is changed by changing energization patterns to the four brushes.Through this configuration, the rotational frequency of the motor ineach mode can be changed while providing a motor in which the number ofmagnetic poles is 4, and multiple slots are achieved.

PRIOR TECHNOLOGY DOCUMENTS Patent Document

-   [Patent Document 1] Japanese Unexamined Patent Application, First    Publication No. 2006-353019

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

Meanwhile, in the above-described motor, gaps are respectively formedbetween four permanent magnets disposed at the yoke. Thus, changes inmagnetic flux increase between the permanent magnet side and the gapswith both circumferential ends of each permanent magnet as a border. Forthis reason, when each tooth passes through both the circumferentialends of each permanent magnet, a magnetic attractive or repulsive forcewhich acts on the tooth changes greatly, and thereby cogging torque isgenerated. As a result, the vibration and noise of the electric motorincreases.

The invention has been made in view of the above-describedcircumstances, and the object thereof is to provide a variable-speedelectric motor and a reduction motor which can reduce vibration andnoise while achieving miniaturization and high performance.

Means for Solving the Problem

The invention relates to an electric motor including a yoke whose numberof pole pairs is 2, and an armature rotationally journalled to the yoke.The armature includes a rotary shaft journalled to the yoke; an armaturecore attached to the rotary shaft and having a plurality of teethextending in a radial fashion toward the radial direction and allowingcoils to be wound therearound, and a plurality of slots formed betweenthe teeth and extending along the axial direction; and a commutatorprovided at the rotary shaft so as to be adjacent to the armature core,and having a plurality of segments arranged in the circumferentialdirection. Brushes which supply electric power to the coils via thesegments come into sliding contact with the segments. In a firstinvention related to the invention, the commutator is provided withshort-circuit members which short-circuit equipotential segments.Additionally, the brushes are constituted by three brushes of alow-speed brush, a high-speed brush, and a common brush used in commonby the low-speed and high-speed brushes and juxtaposed along thecircumferential direction. The circumferential width of the high-speedbrush is set to be smaller than the circumferential width of thelow-speed brush, and the high-speed brush and the low-speed brush areformed so that simultaneous sliding contact with the equipotentialsegments can be avoided. Additionally, the armature cores are providedsuch that the plurality of teeth is point-symmetrical about the rotaryshaft at equal intervals in the circumferential direction, and the teethand slots are formed so as to exist alternately at intervals of 90degrees in the circumferential direction.

In a second invention related to the invention, the number of theplurality of teeth and the plurality of slots are set to any of 7 times,9 times, and 11 times the number of pole pairs.

In a third invention related to the invention, the external diameter ofthe commutator is set within a range of 20 mm or more and 30 mm or less.

In a fourth invention related to the invention, the circumferentialwidths of the low-speed brush and the common brush are set within arange of 2.5 mm or more and 5 mm or less.

In a fifth invention related to the invention, the circumferential widthof the high-speed brush is set to a range which is equal to or more than1.5 mm or more and smaller than 2.5 mm.

Additionally, a sixth invention related to the invention provides areduction motor including the electric motor according to any of theabove first to fifth inventions, a worm shaft coupled to the rotaryshaft of the electric motor, and a worm gear which meshes with the wormshaft.

Effects of the Invention

In the invention, the armature cores are provided such that theplurality of teeth is point-symmetrical about the rotary shaft at equalintervals in the circumferential direction, and the teeth and slots areformed so as to exist alternately at intervals of 90 degrees in thecircumferential direction. Accordingly, in a case where the number ofpole pairs is 2, the relative positional relationship between an N-poleand a tooth and a slot which face this N-pole and the relativepositional relationship between an S-pole and a tooth and a slot whichface this S-pole can be changed.

For this reason, the generation timing of the cogging torque generatedin a tooth (slot) which passes through an N pole, and the generationtiming of the cogging torque generated in a tooth (slot) which passesthrough an S pole can be shifted. Hence, the cogging torque of the wholearmature decreases, and it is possible to reduce the vibration and noiseof the electric motor.

Additionally, by setting the number of the plurality of teeth and theplurality of slots to any of 7 times, 9 times, and 11 times the numberof pole pairs, multiple slots can be formed while reducing the vibrationand noise. For this reason, a high-performance electric motor and areduction motor can be provided.

Moreover, since the brushes are constituted by three brushes of alow-speed brush, a high-speed brush, and a common brush used in commonby the low-speed and high-speed brushes, in respect to making the speedvariable, the number of parts can be reduced as compared to the relatedart.

Moreover, the circumferential width of the high-speed brush is set to besmaller than the circumferential width of the low-speed brush, and thehigh-speed brush and the low-speed brush are formed so as to avoidsimultaneous sliding contact with the equipotential segments. For thisreason, the circumferential width of the segments is decreased comparedto the related art, and miniaturization of the commutator is easilyachieved. As a result, miniaturization of the whole electric motor canbe achieved.

Here, in a case where the electric motor is rotationally driven at lowspeed, as the high-speed brush which is not used comes into slidingcontact with a segment, a difference may be caused in the number ofcoils of an equivalent electric circuit and variation may occur inelectric currents which flow through the respective coils. Thereby,vibration and noise of the electric motor may increase.

However, the influence of the high-speed brush during low rotationaldriving can be made small by setting the circumferential width of thehigh-speed brush to be smaller than the circumferential width of thelow-speed brush. For this reason, an electric motor and a reductionmotor with less vibration and noise can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a reduction motor in an embodiment of theinvention.

FIG. 2 is a longitudinal sectional view of the reduction motor in theembodiment of the invention.

FIG. 3 is a cross-sectional view of an electric motor in the embodimentof the invention.

FIG. 4 is a view as seen from an arrow A of FIG. 2.

FIG. 5 is a developed view of an armature in the embodiment of theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, an embodiment of the invention will be described with reference tothe drawings.

As shown in FIGS. 1 to 3, a reduction motor 1 is used as, for example, awiper motor of an automobile, and includes an electric motor 2, and areduction mechanism 4 coupled to a rotary shaft 3 of the electric motor2.

The electric motor 2 has a bottomed tubular yoke 5, and an armature 6rotatably provided within the yoke 5.

A tubular portion 53 of the yoke 5 is formed substantially in the shapeof a cylinder, and four segment type permanent magnets 7 are disposed atequal intervals in a circumferential direction on the inner peripheralsurface of the tubular portion such that magnetic poles becomealternate. That is, the number of pole pairs of the permanent magnets 7provided at the yoke 5 is set to 2.

The radial center of a bottom wall (end portion) 51 of the yoke 5 isformed with a boss portion 19 which protrudes axially outward, and abearing 18 for journalling one end of the rotary shaft 3 is provided atthe boss portion 19. In addition, the external surface of the yoke 5 ispainted in black. Through this black painting, the amount of heatreleased from the yoke 5 to the outside can be increased, and the risein temperature of the electric motor 2 can be made low.

An opening 53 a of the tubular portion 53 is provided with an outerflange portion 52. The outer flange portion 52 is formed with abolt-hole (not shown). A bolt 24 is inserted through this bolt-hole, andthe yoke 5 is fastened and fixed to the reduction mechanism 4 as thebolt is screwed into a bolt-hole (not shown) formed in a gear housing 23(which will be described below) of the reduction mechanism 4.

The armature 6 is fitted to the rotary shaft 3 from the outside, andincludes a fixed armature core 8, armature coils 9 wound around thearmature core 8, and a commutator 10 arranged at the other end of therotary shaft 3. Each armature core 8 is formed by laminating (laminatedcore) plates made of a magnetic material punched by press working or thelike in the axial direction or pressure-forming (dust core) softmagnetic powder, and has a substantially annular core body 11.

Eighteen teeth 12, which are substantially T-shaped in an axial planview, are provided in a radial fashion at equal intervals along thecircumferential direction at an outer peripheral portion of the corebody 11. Each tooth 12 is constituted by a winding drum portion 31 whichextends radially and has a winding wire 14 wound therearound, and aperipheral wall portion 32 which is provided at the tip of the windingdrum portion 31 and extends so as to become symmetrical with respect tothe winding drum portion 31. That is, the peripheral wall portion 32provided at the tip of the tooth 12 constitutes the outer peripheralsurface of the armature core 8, and the peripheral wall portion 32 isbrought into a state where the peripheral wall portion faces a permanentmagnet 7.

Eighteen dovetail slots 13 are provided between adjacent teeth 12 byproviding the teeth 12 in a radial fashion at the outer peripheralportion of the core body 11. The slots 13 extend along the axialdirection, and are formed at equal intervals along the circumferentialdirection.

An enamel-coated winding wire 14 is inserted through between the slots13, and the winding wire 14 is wound around the winding drum portion 31of the tooth 12 via an insulator (not shown) which is an insulatingmaterial. Thereby, a plurality of armature coils 9 is formed on theouter periphery of the armature core 8.

Here, when the eighteen teeth 12 are formed at equal intervals along thecircumferential direction, all the teeth 12 and slots 13 existpoint-symmetrically about the rotary shaft 3. On the other hand, theteeth 12 and the slots 13 exist alternately in a positional relationshipwith intervals of 90 degrees in the circumferential direction.

That is, since four permanent magnets 7 (the number of magnetic poles isfour) are provided in the present embodiment, the number of pole pairsis two, and the eighteen teeth 12 (slots 13) are provided with respectto this. That is, the number of the teeth 12 (slots 13) is set to 9times the number of pole pairs.

Additionally, all the teeth 12 and slots 13 are point-symmetrical aboutthe rotary shaft 3, and the teeth 12 and the slots 13 exist alternatelyin a positional relationship with intervals of 90 degrees in thecircumferential direction. Thereby, the relative positional relationshipbetween an N-pole permanent magnet 7 and each tooth 12 which faces thismagnet and the relative positional relationship between an S-polepermanent magnet 7 and each tooth 12 which faces this magnet are broughtinto the state of having shifted by a distance equivalent to half of thewidth of a slot 13 along the circumferential direction.

The commutator 10 is inserted from the outside into and fixed closer tothe other end of the rotary shaft 3 than the armature core 8. Eighteensegments 15 formed from a conductive material are attached to the outerperipheral surface of the commutator 10. The segments 15 are made of aplate-like metal piece which is long in the axial direction, and arefixed in parallel at equal intervals along the circumferential directionin a state where the segments are insulated from each other. Theexternal diameter D1 of the commutator 10 is set within a range of 20 mmor more and 30 mm or less.

A riser 15 which is bent in a folded-back fashion to theexternal-diameter side is integrally formed at the end of each segment15 on the side of the armature core 8. A winding wire 14 which becomes awinding starting end and a winding finishing end of an armature coil 9is hung around the riser 16, and the winding wire 14 is fixed to theriser 16 by fusing or the like. Thereby, a segment 15 and an armaturecoil 9 corresponding to this segment are electrically connected to eachother.

Additionally, connecting wires 40 are respectively hung around therisers 15 corresponding to equipotential segments 15, i.e., segments 15(every pair of segments 15 separated from nine positions in the presentembodiment) which face each other about the rotary shaft 3, and theconnecting wires 40 are fixed to the risers 16 by fusing or the like(refer to FIG. 5). The connecting wires 40 are provided to short-circuitthe equipotential segments 15 from each other, and are disposed betweenthe commutator 10 and the armature core 8.

The commutator 10 configured in this way is brought into a state wherethe commutator faces the gear housing 23 of the reduction mechanism 4.The gear housing 23 is constituted by a housing body 42 which is formedsubstantially in the shape of a box having an opening 42 a on one facethereof and houses a gear group 41 of the reduction mechanism 4, and acover 43 which blocks the opening 42 a of the housing body 42. A brushhousing portion 22 is formed on the electric motor 2 side of the housingbody 42, and the commutator 10 of the electric motor 2 faces the brushhousing portion.

As shown in FIGS. 2 to 4, the brush housing portion 22 is concavelyformed on the electric motor 2 side of the gear housing 23. A peripheralwall 30 of the brush housing portion 22 is formed so as to have asubstantially oval cross-section, and is constituted by planar walls 30a and arcuate walls 30 b.

A cover 33, which is formed in the shape of a tube having asubstantially oval cross-section so as to correspond to the brushhousing portion, is provided inside the brush housing portion 22. Thecover 33 also has planar walls 33 a and arcuate walls 33 b. Moreover, aholder stay 34 formed so as to correspond to the cover 33 is providedinside the cover 33. The holder stay 34 is fastened and fixed to theside wall 42 b of the housing body 42 by bolts 35.

Brush holders 36 are provided in three places along the circumferentialdirection at the holder stay 34. Brushes 21 are supported within thebrush holders 36, respectively, so as to protrude and retract from thebrush holders in a state where the brushes are biased via springs S,respectively. Since the tips of the brushes 21 are biased by the springsS, the tips of the brushes come into sliding contact with the commutator10, and the electric power (not shown) from the outside is supplied tothe commutator 10 via the brushes 21.

Brushes 21 are constituted by a low-speed brush 21 a and a high-speedbrush 21 b, which are connected to the anode side, and a common brush 21c which is used in common by the low-speed brush 21 a or the high-speedbrush 21 b, and is connected to the cathode side. The low-speed brush 21a and the common brush 21 c are mutually disposed at an interval of 180°in electrical angle, i.e., at an interval of 90° in the circumferentialdirection in mechanical angle. Meanwhile, the high-speed brush 21 b isarranged apart from the low-speed brush 21 a by an angle α in thecircumferential direction. In addition, although the present embodimentdescribes that the common brush 21 c is used as the cathode side, andthe low-speed brush 21 a and the high-speed brush 21 b are used as theanode side, the anode side and cathode side may be reversed.

In the present embodiment, the electric resistance value of thehigh-speed brush 21 a is set to be two or more times higher than theelectric resistance values of the low-speed brush 21 a and the commonbrush 21 c. Therefore, the current value when an electric current issupplied to the armature coils 9 from the high-speed brush 21 a can belowered. Thereby, when an electric current is supplied to the armaturecoils 9 from the high-speed brush 21 a and the armature 6 of theelectric motor 2 is rotating at high speed, the current value of a largecurrent (lock current) supplied to the armature coils 9 can be loweredin a case where the rotation of the armature 6 is stopped (locked) by anexternal load. Therefore, any damage to an element for protecting anelectric circuit, such as a fuse provided in a motor drive device, canbe prevented in advance.

Here, since the equipotential segments 15 of the commutator 10, i.e.,the segments 15 which face each other about the rotary shaft 3, areshort-circuited by the connecting wires 40, it is possible to supplyelectric power even to the segments with which the brushes 21 do notcome into sliding contact. Accordingly, the high-speed brush 21 b existsat a position which is advanced by an angle θ with respect to thelow-speed brush 21 a. In addition, in the present embodiment, the angleθ is set to 30 degrees.

By arranging the respective brushes 21 a to 21 c in this way, theportions of the cover 33 and the holder stay 34 where the brushes 21 ato 21 c do not exist can be cut away. That is, the cover 33 can beformed so as to have a substantially oval cross-section, and thelow-speed brush 21 a and the common brush 21 c can be arranged nearconnecting portions between the planar walls 33 a and the arcuate wall33 b. On the other hand, the high-speed brush 21 b can be arranged atthe arcuate wall 33 b of the cover 33 opposite to places, in which thelow-speed brush 21 a of the cover 33 and the common brush 21 c arearranged, about the rotary shaft 3. For this reason, the brush housingportion 22 can be formed so as to have a substantially ovalcross-section, and it is possible to flatten the brush housing portion22.

Additionally, as shown in detail in FIG. 3, the circumferential brushwidths W1 by which the low-speed brush 21 a and the common brush 21 ccome into sliding contact with the commutator 10 are set to be almostthe same. In contrast, the circumferential brush width W2 by which thehigh-speed brush 21 b comes into sliding contact with the commutator 10is set to be smaller than the brush width W1 of the low-speed brush 21a. Specifically, when the external diameter of the commutator 10 is setwithin a range of 20 mm or more and 30 mm or less, the brush widths W1of the low-speed brush 21 a and the common brush 21 c are set within arange of 2.5 mm or more and 5 mm or less. On the other hand, the brushwidth W2 of the high-speed brush 21 b is set to a range which is equalto and more than 1.5 mm and smaller than 2.5 mm.

By setting the brush widths W1 of the low-speed brush 21 a and thecommon brush 21 c, and the brush width W2 of the high-speed brush 21 bin this way, it is possible to avoid a situation where the low-speedbrush 21 a and the high-speed brush 21 b come into sliding contact withthe same segment 15 simultaneously.

That is, for example, the low-speed brush 21 a exists even at a positionpoint-symmetrical with respect thereto about the rotary shaft 3 by aconnecting wire 40 connected to the commutator 10 (refer to a two-dotchain line in FIG. 3). In this case, the spacing between the low-speedbrush 21 a and the high-speed brush 21 b becomes almost the same as thespacing between adjacent segments 15, 15. However, since the brush widthW2 of the high-speed brush 21 b is set to be smaller than the brushwidth W1 of the low-speed brush 21 a, it is possible to avoid asituation where the low-speed brush 21 a and the high-speed brush 21 bcome into sliding contact with the same segment 15 simultaneously.

This is also the same between the high-speed brush 21 b and the commonbrush 21 c. That is, the high-speed brush 21 b exists even at a positionpoint-symmetrical with respect thereto about the rotary shaft 3 by aconnecting wire 40 connected to the commutator 10. However, since thebrush width W2 of the high-speed brush 21 b is set to be smaller thanthe brush width W1 of the common brush 21 c, it is possible to avoid asituation where the high-speed brush 21 b and the common brush 21 c comeinto sliding contact with the same segment 15 simultaneously.

As shown in FIGS. 1 and 2, the gear group 41 housed in the housing body42 of the gear housing 23 is constituted by a worm shaft 25 coupled tothe rotary shaft 3 of the electric motor 2, a stepped gear 26 whichmeshes with the worm shaft 25, and a spur gear 27 which meshes with thestepped gear 26. The worm shaft 25 has one end coupled to the rotaryshaft 3 and the other end rotationally journalled to the housing body42. The stepped gear 26 is obtained by integrally forming a worm gear 28which meshes with the worm shaft 25, and a smaller-diameter gear 29which is formed to have a smaller diameter than the worm gear 28.

An idler shaft 61 is press-fitted into the radial center of the steppedgear 26. The idler shaft 61 protrudes to the side opposite to thesmaller-diameter gear 29, and this protruding end 61 a is rotationallyjournalled to the housing body 42. On the other hand, the tip of thesmaller-diameter gear 29 which exists at the end opposite to the end 61a of the idler shaft 61 is rotationally journalled to the cover 43. Inthis way, the stepped gear 26 is brought into a state where both endsthereof are journalled to the housing body 42 and the cover 43.

The spur gear 27 meshes with the smaller-diameter gear 29 of the steppedgear 26. A boss portion 65 is formed at the radial center of the spurgear 27 so as to protrude toward the cover 43 side. The boss portion 65is rotationally supported by the cover 43. Additionally, an output shaft62 is press-fitted into the boss portion 65. The output shaft 62protrudes from a bottom wall (end portion) 42 c of the housing body 42.The boss portion 63 is formed at the part of the bottom wall 42 c of thehousing body 42 corresponding to the output shaft 62 so as to protrudeoutward. The boss portion 63 is provided with a sliding bearing 64 forrotationally journalling the output shaft 62.

The portion of the output shaft 62 which protrudes from the housing body42 is formed with a tapered portion 66 which is gradually tapered as itgoes to the tip. The tapered portion 66 is formed with serrations 67.Thereby, for example, an external mechanism for driving a wiper or thelike and the output shaft 62 can be coupled together.

In addition, a connector 68 is provided at the side wall 42 b of thehousing body 42 so as to protrude along the axial direction of therotary shaft 3. The connector 68 is provided to supply the electricpower from the outside to the electric motor 2. A receiving port 69 ofthe connector 68 is provided with a connection terminal 70, and theconnection terminal 70 is electrically connected to the brushes 21 (21 ato 21 c) of the electric motor 2. Therefore, the electric power from theoutside is supplied to the commutator 10 via the brushes 21.

Moreover, a bolt seat 71 for fastening and fixing the cover 43 is formedintegrally with an opening edge of the housing body 42. An attachmentseat 73, which has a bolt-hole (not shown) through which the bolt 72 canbe inserted, is integrally formed at the part of the housing body 42 ofthe cover 43 corresponding to the bolt seat 71. As the bolt 72 isinserted through the attachment seat 73, and the bolt 72 is screwed intothe bolt seat 71 of the housing body 42. Thereby, the cover 43 isfastened and fixed to the housing body 42.

Additionally, the cover 43 is provided with a power distributionsubstrate 74 for electrically connecting the connection terminal 70 ofthe connector 68 and the brushes 21 of the electric motor 2. The powerdistribution substrate 74 is formed with a pattern (not shown) which hasa role of a lead wire.

Next, the structure for winding the winding wire 14 around the armaturecore 8 of the armature 6 will be described with reference to FIG. 5.

FIG. 5 is a developed view of the armature 6, and a gap between adjacentteeth 12 is equivalent to the slot 13. In addition, in the followingdrawings, the respective segments 15 and the respective teeth 12 will bedescribed with reference numerals given thereto.

As shown in detail in this drawing, the equipotential segments 15 areshort-circuited by the connecting wires 40. That is, in the presentembodiment, every ninth segment 15 (for example, a first segment 15 anda tenth segment 15) are short-circuited by the connecting wires 40,respectively.

Here, the winding wire 14 is constituted by a first conductive wire 110and a second conductive wire 120. In addition, in FIG. 5, the firstconductive wire 110 is shown by solid lines, and the second conductivewire 120 is shown by broken lines.

The first conductive wire 110 is wound in the forward direction betweenevery pair of arbitrary slots 13 and 13 separated by three positions,and is wound in the backward direction between slots 13 and 13 whichhave shifted by 90 degrees, respectively, in the circumferentialdirection from the arbitrary slots 13 and 13, thereby forming first toninth winding wires 161 to 169.

The second conductive wire 120 is wound in the forward direction betweenevery pair of arbitrary slots 13 and 13 separated by three positions,and is wound in the backward direction between slots 13 and 13 whichhave shifted by 90 degrees, respectively, in the circumferentialdirection from the arbitrary slots 13 and 13, thereby forming otherfirst to ninth winding wires 171 to 179.

The first to ninth winding wires (161 to 169) and the other first toninth winding wires (171 to 179) exist at positions which arerespectively point-symmetrical about the rotary shaft 3.

That is, the first winding wire 161 formed by the first conductive wire110 and the other first winding wire 171 formed by the second conductivewire 120 exist at positions which face each other about the rotary shaft3, and a first winding wire pair is formed by the winding wire 161 andthe winding wire 171. Similarly, a second winding wire pair (162, 172),a third winding wire pair (163, 173), a fourth winding wire pair (164,174), a fifth winding wire pair (165, 175), a sixth winding wire pair(166, 176), a seventh winding wire pair (167, 177), an eighth windingwire pair (168, 178), and a ninth winding wire pair (169, 179) areformed by the second to ninth winding wires 162 to 169 and the othersecond to ninth winding wires 172 to 179, respectively.

The first to ninth winding wires 161 to 169 are connected in a seriesvia nine connecting wires 40, respectively. On the other hand, the otherfirst to ninth winding wires 171 to 179 are connected in a series vianine connecting wires 40, respectively. A winding starting end and awinding finishing end of each of the winding wires 161 to 179 areconnected between adjacent segments 15 and 15. The first to ninthwinding wires 161 to 169 and the other first to ninth winding wires 171to 179, which are formed between the slots 13, respectively, in thisway, can be wound, for example, using a double flyer type windingmachine or the like.

Next, the operation of the reduction motor 1 will be described.

First, during low rotational driving, in the electric motor 2 of thereduction motor 1, electric power is supplied to the common brush 21 cand the low-speed brush 21 a. At this time, magnetic fields aregenerated in the armature coils 9 wound around the armature core 8, andmagnetic attractive or repulsive forces are generated between themagnetic fields and the permanent magnets 7 provided at the yoke 5 todrive the rotary shaft 3. On the other hand, during high rotationaldriving, the electric motor 2 is advanced by the high-speed brush 21 b,and operates at higher rotational speed than that during low rotationaldriving.

When the rotary shaft 3 is driven, the driving is transmitted to theoutput shaft 62 via the reduction mechanism 4. Since an externalmechanism for driving a wiper or the like is coupled to the output shaft62, the external mechanism operates at a low speed or operates at a highspeed according to rotation of the output shaft 62.

Additionally, all the teeth 12 and slots 13 of the electric motor 2 arepoint-symmetrical about the rotary shaft 3, and the teeth 12 and theslots 13 exist alternately in a positional relationship with intervalsof 90 degrees in the circumferential direction, i.e., at an interval of180 degrees in electrical angle. Thereby, the relative positionalrelationship between an N-pole permanent magnet 7 and each tooth 12which faces this magnet and the relative positional relationship betweenan S-pole permanent magnet 7 and each tooth 12 which faces this magnetare brought into the state of having deviated by a distance equivalentto half of the width of a slot 13 along the circumferential direction.

For this reason, the generation timing of the cogging torque of a tooth12 of the portion corresponding to an N pole and the generation timingof the cogging torque of a tooth 12 of the portion corresponding to an Spole shift from each other. Therefore, the cogging torque of the wholeelectric motor 2 decreases.

Additionally, since electric power is supplied by the common brush 21 cand the low-speed brush 21 a during low rotational driving, electricpower is not supplied to the high-speed brush 21 b, and a non-energizedstate is brought about. For this reason, when the high-speed brush 21 bexists so as to straddle between adjacent segments 15 and 15, thesegments 15 and 15 are short-circuited by the high-speed brush 21 b.Also, the winding wire 14 connected to the short-circuited segments 15and 15 only becomes a closed loop. As a result, a difference is causedin the number of coils of an equivalent electric circuit.

At this time, since magnetic flux passes through the winding wire 14which becomes a closed loop by the high-speed brush 21 b, an inducedvoltage (counter-electromotive force) is generated in the winding wire14 due to a change in this magnetic flux. An electric current in adirection opposite to the energized direction flows momentarily due tothis induced voltage (counter-electromotive force), and commutationdeteriorates. This will increase a torque ripple.

However, in the present embodiment, the brush widths W1 of the low-speedbrush 21 a and the common brush 21 c in the circumferential directionare set to be almost the same as each other, and the circumferentialbrush width W2 of the high-speed brush 21 b is set to be smaller thanthe brush width W1 of the low-speed brush 21 a (refer to FIG. 3). Forthis reason, the time for which the high-speed brush 21 b short-circuitsadjacent segments 15 and 15 can be set, and the electric current whichflows into the winding wire 14 in which a closed loop has been formed bythe high-speed brush 21 b, can be reduced.

On the other hand, since electric power is supplied by the common brush21 c and the high-speed brush 21 b during high rotational driving,electric power is not supplied to the low-speed brush 21 a, and anon-energized state is brought about. However, since an induced voltage(counter-electromotive force) which deteriorates commutation is noteasily generated in the low-speed brush 21 a, the low-speed brush 21 ahas almost no influence during high rotational driving.

Accordingly, according to the above-described embodiment, the number ofpole pairs is 2, i.e., the number of magnetic poles are four, andmultiple slots are formed (the numbers of slots are 7 times, 9 times,and 11 times the number of pole pairs). Thus, the cogging torque can bereduced even in the variable-speed electric motor 2. For this reason, itis possible to reduce the vibration and noise of the electric motor 2(reduction motor 1).

Particularly, an increase in the torque ripple resulting from thehigh-speed brush 21 b can be reduced during low rotational driving withhigh use frequency, as compared to during high rotational driving. Forthis reason, it is possible to further reduce the vibration and noise ofthe electric motor 2 during low rotational driving.

Additionally, by setting the circumferential brush width W2 of thehigh-speed brush 21 b to be smaller than the circumferential brushwidths W1 of the low-speed brush 21 a and the common brush 21 c, it ispossible to avoid a situation where the low-speed brush 21 a and thehigh-speed brush 21 b come into sliding contact with the same segment 15simultaneously. For this reason, it is possible to provide a small-sizedhigh-performance electric motor 2 with multiple slots.

Moreover, as the reduction motor 1 is constituted by the electric motor2, and the reduction mechanism 4 coupled to the rotary shaft 3 of theelectric motor 2, it is possible to achieve miniaturization and highperformance of the reduction motor 1.

In addition, it should be understood that the invention is not limitedto the above-described embodiment, but various modifications may be madeto the above-described embodiment without departing from the scope ofthe invention.

Additionally, the case where the armature core 8 of the electric motor 2is provided with eighteen teeth 12, and the number of the teeth 12 isset to 9 times the number of pole pairs has been described in theabove-described embodiment. However, the number of the teeth 12 is notlimited thereto. In a case where high performance by multiple slots ofthe electric motor 2 is taken into consideration, it is desirable to setthe number of the teeth 12 to any of 7 times, 9 times, and 11 times thenumber of pole pairs.

INDUSTRIAL APPLICABILITY

As described above, according to the invention, it is possible toprovide a variable-speed electric motor and a reduction motor which canreduce the vibration and noise while achieving miniaturization and highperformance.

DESCRIPTION OF THE REFERENCE SYMBOLS

-   -   1: REDUCTION MOTOR    -   2: ELECTRIC MOTOR    -   3: ROTARY SHAFT    -   4: REDUCTION MECHANISM    -   5: YOKE    -   6: ARMATURE    -   7: PERMANENT MAGNET (MAGNETIC POLE)    -   8: ARMATURE CORE    -   9: ARMATURE COIL (COIL)    -   10: COMMUTATOR    -   12: TEETH    -   13: SLOT    -   14: WINDING WIRE (COIL)    -   15: SEGMENT    -   21: BRUSH    -   21 a: LOW-SPEED BRUSH    -   21 b: HIGH-SPEED BRUSH    -   21 c: COMMON BRUSH    -   25: WORM SHAFT    -   28: WORM GEAR    -   40: CONNECTING WIRE (SHORT-CIRCUITING MEMBER)    -   D1: EXTERNAL DIAMETER    -   W1, W2: BRUSH WIDTH

1. An electric motor comprising: a yoke whose number of pole pairs is 2;and an armature rotationally journalled to the yoke, the armatureincluding: a rotary shaft journalled to the yoke; an armature coreattached to the rotary shaft and having a plurality of teeth extendingin a radial fashion toward the radial direction and allowing coils to bewound therearound, and a plurality of slots formed between the teeth andextending along the axial direction; a commutator provided at the rotaryshaft so as to be adjacent to the armature core, and having a pluralityof segments arranged in the circumferential direction, wherein brusheswhich supply electric power to the coils via the segments come intosliding contact with the segments, wherein the commutator is providedwith short-circuit members which short-circuit equipotential segments,wherein the brushes are constituted by three brushes of a low-speedbrush, a high-speed brush, and a common brush used in common by thelow-speed and high-speed brushes and juxtaposed along thecircumferential direction, wherein the circumferential width of thehigh-speed brush is set to be smaller than the circumferential width ofthe low-speed brush, and the high-speed brush and the low-speed brushare formed so that simultaneous sliding contact with the equipotentialsegments can be avoided, and wherein the armature cores are providedsuch that the plurality of teeth is point-symmetrical about the rotaryshaft at equal intervals in the circumferential direction, and the teethand the slots are formed so as to exist alternately at intervals of 90degrees in the circumferential direction.
 2. The electric motoraccording to claim 1, wherein the number of the plurality of teeth andthe plurality of slots are set to any of 7 times, 9 times, and 11 timesthe number of pole pairs.
 3. The electric motor according to claim 1,wherein the external diameter of the commutator is set to 20 mm or moreand 30 mm or less.
 4. The electric motor according to claim 1, whereinthe circumferential widths of the low-speed brush and the common brushare set to 2.5 mm or more and 5 mm or less.
 5. The electric motoraccording to claim 1, wherein the circumferential width of thehigh-speed brush is set to a range which is equal to or more than 1.5 mmor more and smaller than 2.5 mm.
 6. A reduction motor comprising: theelectric motor according to any of claims 1 to 5, a worm shaft coupledto the rotary shaft of the electric motor, and a worm gear which mesheswith the worm shaft.